1. Field of the Invention
The present invention relates to a sheet conveying device including a sheet reversing mechanism capable of reversing a sheet, and to an image forming apparatus, such as a copy machine, a printer, and a facsimile machine, including the sheet conveying device.
2. Description of the Related Art
In general, a sheet conveying device for an image forming apparatus including a sheet reversing mechanism uses a switch-back method. In the switch-back method, a sheet is pulled into a reversing conveying path and is then extracted in the opposite direction to reverse the sheet. In the sheet conveying device having the sheet reversing mechanism using the switch-back method, the reversing conveying path dedicated to reversing the sheet and a switch for switching conveying paths are required. This leads to an increase in the size of the sheet conveying device and the image forming apparatus.
Accordingly, to reduce the size of the sheet conveying device and the image forming apparatus, a sheet reversing mechanism that reverses a sheet by rotating a pair of planetary rollers while the sheet is nipped therebetween has been suggested. This technique is described in Japanese Patent Laid-Open No. 2001-122479.
The structure of known planetary rollers will be described below with reference to FIGS. 8A to 8C.
A pair of planetary rollers 102 and 103 is used to reverse and convey a small, thick sheet, such as a card. The planetary rollers 102 and 103 are provided on a rotating disc 120 having a diameter corresponding to the length of a side of a card-shaped sheet (hereinafter simply called a sheet) 101 in a conveying direction thereof. The planetary rollers 102 and 103 include a sun roller 102 that functions as a driving roller and that is rotated by a driving force, and a satellite roller 103 that functions as a driven roller and that is capable of holding the sheet 101 between the sun roller 102 and the satellite roller 103. Guides 140 for guiding the sheet 101 toward the planetary rollers 102 and 103 are attached to the rotating disc 120. The rotating disc 120 is supported such that the rotating disc 120 can rotate around a drive shaft of the sun roller 102.
A pair of supply rollers 121 for conveying the sheet 101 to the position between the sun roller 102 and the satellite roller 103 and guides 140 for guiding the sheet 101 are disposed around the rotating disc 120. In addition, a card position sensor 131 for detecting the trailing edge of the sheet 101 supplied by the supply rollers 121 and a rotational angle sensor 132 for detecting the rotational angle of the rotating disc 120 are also disposed around the rotating disc 120.
A sheet-reversing operation performed by the known planetary rollers 102 and 103 having the above-described structure will now be described.
The sun roller 102 is rotated at the same surface speed (peripheral speed) as that of the supply rollers 121. When the sheet 101 is supplied, the rotating disc 120 is stopped at the position shown in FIG. 8A. In this state, the sheet 101 is conveyed by the supply rollers 121 in the direction shown by the arrow X. After the leading edge of the sheet 101 reaches the position where it is nipped between the sun roller 102 and the satellite roller 103, the sheet 101 is further conveyed while being nipped by the planetary rollers 102 and 103. Then, when it is detected by the card position sensor 131 that the trailing edge of the sheet 101 has passed a predetermined position and the overall body of the sheet 101 is supported by the rotating disc 120, the rotating disc 120 starts to rotate together with the sun roller 102 (see FIG. 8B).
When it is detected by the rotational angle sensor 132 that the rotating disc 120 has rotated about 180° and reached the position shown in FIG. 8C, the rotating disc 120 stops rotating. At this time, the sun roller 102 continues to rotate, so that the sheet 101 nipped between the sun roller 102 and the satellite roller 103 is ejected in the direction shown by the arrow Y. The supply rollers 121 are rotated in the direction opposite to that in the state shown in FIG. 8A. Accordingly, the sheet 101 is conveyed by the supply rollers 121 after being reversed by the rotation of the rotating disc 120.
In the above-described known technique, the sheet 101 is rotated around the drive shaft of one of the planetary rollers 102 and 103, i.e., the sun roller 102, while the sheet 101 is nipped between the planetary rollers 102 and 103. Therefore, the sheet 101 cannot be conveyed along a straight conveying path before and after the rotation, but is conveyed along paths that are shifted from each other. In other words, the position at which the sheet 101 is received by the planetary rollers 102 and 103 and the position at which the sheet 101 is ejected therefrom are shifted from each other in the vertical direction in FIGS. 8A to 8C. Therefore, the sheet conveying performance is degraded and the occurrence of jamming is increased.